1. Field of the Invention
This invention relates to an improved method of continuously preparing a fracturing fluid or the like during the well treatment on a real time basis. More specifically, but not by way of limitation, the invention relates to a method for rapid hydration of a polymer phase dispersed in an oil based fluid.
2. Description of the Prior Art
High viscosity fracturing fluids are often formulated using dry additives which are mixed with water or other aqueous fluids at the job site. Such commercial mixing procedures are known to involve inherent problems, particularly on remote sites or when large volumes are required. For example, special equipment for mixing the dry additives with water is required and problems such as chemical dusting, uneven mixing, lumping of the gels while mixing and extended preparation and mixing time are involved. The mixing and physical handling of large quantities of dry chemicals require a great deal of manpower, and when continuous mixing is required, the accurate and efficient handling of dry chemicals is extremely difficult. Also, with respect to batch mix applications, the occurrence of job delays can result in the deterioration of premixed gels and potential loss thereof as well as chemical losses due to tank bottoms and problems associated with the cost of pretreatment tank clean up.
In a commonly assigned copending application Ser. No 070,082, a fracturing fluid slurry concentrate useful in a continuous process for supplying a viscous fracturing fluid for hydraulic fracturing of subterranean formations is disclosed. Such a slurry concentrate typically involves a polymer slurry wherein a hydratable polymer is dispersed in a hydrophobic solvent (hereinafter referred to as an oil based fluid) in combination with a suspension agent and a surfactant with or without other optional additives commonly employed in well treatment applications. Because of the inherent dispersion of the hydratable polymer in the oil based fluid (i.e., lack of affinity for each other), such fracturing fluid slurry concentrates tend to eliminate lumping and premature gelation problems and tend to optimize initial dispersion when added to water. However, the rate of hydration of the polymer is still a critical factor particularly in continuous mix applications wherein the necessary hydration and associated viscosity rise must take place over a relatively short time span corresponding to the residence time of the fluids during the continuous mix procedure.
In such applications, hydration is the process by which the hydratable polymer absorbs water. Once the polymer is dispersed, its ability to absorb water will dictate hydration or hydration rate. Several factors will determine how readily the polymer will hydrate or develop viscosity; e.g., the pH of the system (particularly for natural polymers), the amount of mechanical shear applied in the initial mixing phase, the concentration of salts and the polymer concentration. Hydration rate can be influenced through pH control agents which may be blended with the polymer or added to the aqueous medium. Hydration rate can also be controlled by the level of applied shear, with the solution viscosity increasing faster when subjected to high shear. The rate of viscosity development may be influenced (particularly in low shear applications) bythe salts present in the solution. The extent of retardation is dependent on the concentration and type of salt. Finally, the viscosity level achieved at a particular point in time is a function of polmer concentration.
Unmodified guar will develop viscosity in all electrolyte systems such as those containing KCl, NaCl, and CaCl.sub.2 at high concentrations. Guar gum hydrates most efficiently in the pH range of 7-8, yielding viscosities of 32-36 cps at 511 sec.sup.-1 in 2% KCl. Guar will not hydrate in organic solvents such as methanol.
Hydroxypropyl guar (HPG) hydrates well in many salt systems at 80.degree. F., and also develops excellent viscosity at temperatures of 40.degree. F. Depending on the mechanical shear applied, 80-90% of the viscosity can be achieved within ten minutes. Optimum hydration of HPG can be realized in the pH range of 4-6. HPG also viscosifies mixtures of methanol and 2% KCL water used typically in a ratio of 50:50.
Carboxymethyl hydroxypropyl guar (CMHPG) hydrates in most electrolyte make-up solutions, however, it is more sensitive to these solutions than guar and HPG. CMHPG hydrates well in both cold and warm water.
In contrast to the above natural polymers, synthetic polymers may also be dispersed and hydrated; however, they may not be as sensitive to pH effects. Consequently hydration and dispersion will rely more on mechanical mixing for these polymers.
Several attempts have been made over the last 30 years to perfect a process and chemicals for continuous preparation of fracturing fluids. A continuous process would allow the fluids to be made "real time" during the fracturing process. This process would have several advantages over the current common method of producing fluids which involve "batch" mixing of water, gelling agent, and other additives into individual "frac" tanks before the treatment is begun. This process is expensive because of the time and equipment required and because of wasted and unused fluids resulting from treatment delays, termination of the treatment before pumping all fluids, and fluid left in the bottom of tanks which cannot be pumped out. The disposal of unused fluids has also become an expensive process because of stricter laws on the disposal of chemical wastes.